Typical buck regulator power converters, such as that illustrated in FIG. 1, utilize two switches 10 and 20 (e.g. MOSFETs) to connect the converter's switch node (Vx) to either Vin or to ground. The switch node can present a pulse-width modulated square wave with its high-level at the voltage of Vin and it's low-level at the ground voltage of the input of a second order output filter 30. The output filter time-averages the switch node square wave to present a filtered output voltage that is proportional to the amount of time per cycle the switch node is connected to Vin.
Each switch in this typical configuration must be able to block the full input voltage (the difference between the voltage at Vin and ground) while off and while turning on. Therefore, each switch must have a minimum breakdown voltage equal to the input voltage plus a margin. In practice, it is typical to have the minimum breakdown voltage equal to two times the input voltage in discrete circuits where parasitic reactance in the interconnect causes destructive voltage spiking. This may be less severe in fully integrated regulators.
MOSFET switch performance characteristics are dramatically impacted by their breakdown voltage. Two critical performance metrics in a MOSFET are its RDSon and gate charge (Qg). In general, MOSFET RDSon per unit area is proportional to the square of its breakdown voltage. In addition, Qg is proportional to the area of gate and the thickness of the oxide underneath the gate. Higher voltage MOSFETs typically feature a thicker gate oxide which increases the Qg. This is compounded by the aforementioned fact that the gate area of a higher voltage MOSFET must be exponentially larger to achieve the same RDSon as a lower voltage version, thus exponentially increasing Qg as well.
Since losses in a MOSFET are proportional to both RD Son and Qg, reducing the blocking requirements on the MOSFETs in a voltage regulator is highly advantageous.